1. Field of the Invention
The present invention relates to an improved susceptor which inhibits the deposition of process gasses on the edge and backside of a substrate, and which may be easily removed and cleaned.
2. Description of the Related Art
Chemical vapor deposition (CVD) is one of a number of processes used to deposit thin films of material on semiconductor substrates. To process substrates using CVD, a vacuum chamber is provided with a susceptor configured to receive a substrate. In a typical CVD chamber, the substrate is placed into and removed from the chamber by a robot blade and is supported by a substrate support during processing. A precursor gas is charged into the vacuum chamber through a gas manifold plate situated above the substrate, where the substrate is heated to process temperatures, generally in the range of about 250° to 650° C. The precursor gas reacts on the heated substrate surface to deposit a thin layer thereon and to form volatile byproduct gases, which are pumped away through the chamber exhaust system.
A primary goal of substrate processing is to obtain the largest useful surface area, and as a result the greatest number of chips, possible from each substrate. This is highlighted by the recent demands from semiconductor chip manufacturers to minimize edge exclusion on the substrates processed, so that as little of the substrate surface as possible, including the edge of the wafer, is wasted. Some important factors to consider include processing variables that affect the uniformity and thickness of the layer deposited on the substrate, and contaminants that may attach to the substrate and render all or a portion of the substrate defective or useless. Both of these factors should be controlled to maximize the useful surface area for each substrate processed.
One source of particle contamination in the chamber is material deposited at the edge or on the backside of the substrate that flakes off or peels off during a subsequent process. Substrate edges are typically beveled, making deposition difficult to control over these surfaces. Thus, deposition at substrate edges is typically nonuniform and, where metal is deposited, tends to adhere differently to a dielectric than to silicon. If a wafer's dielectric layer does not extend to the bevel, metal may be deposited on a silicon bevel and eventually chip or flake, generating unwanted particles in the chamber. Additionally, chemical mechanical polishing is often used to smooth the surface of a substrate coated with tungsten or other metals. The act of polishing may cause any deposits on the edge and backside surfaces to flake and generate unwanted particles.
A number of approaches have been employed to control the deposition on the edge of the substrate during processing. One approach employs a shadow ring which essentially masks a portion of the perimeter of the substrate from the process gasses. One disadvantage with the shadow ring approach is that, by masking a portion of the substrate's perimeter, the shadow ring reduces the overall useful surface area of the substrate. This problem is made worse if the shadow ring is not accurately aligned with the substrate, and alignment can be difficult to achieve.
Another approach employs a purge ring near the edge of the substrate for delivering a purge gas along the substrate's edge to prevent edge deposition. The purge gas limits or prevents the deposition gas from reaching the substrate and thus limits or prevents deposition on the wafer's beveled edge. A third approach uses a shutter ring and a purge ring in combination to form a purge gas chamber having a purge gas inlet and outlet adjacent the substrate's edge so as to guide the purge gas across the wafer's edge.
A wafer typically sits inside (radially) the purge ring, with a gap therebetween. Conventionally, purge rings are made of aluminum and are welded to the substrate support in an effort to prevent the ring from deforming during processing. However, during the thermal cycling which occurs within a CVD processing chamber, the aluminum rings nonetheless deform, losing the integrity of their shape and therefore compromise their ability to keep particles from depositing on the substrate's edge. This can change the size of the gap, leading to non-uniformity of deposition across the wafer's edge. As the aluminum rings expand and contract, material thereon can flake, and create particles which can contaminate the wafer.
Further, in order for the rings to work effectively for shadowing and/or for purging, they must be frequently cleaned to remove deposition material which can alter the gap or flake off and contaminate the wafer. Such cleaning increases chamber downtime, reduces throughput and results in higher operating costs.
Accordingly a need exists for an improved susceptor which can reliably prevent edge deposition, and which can be easily cleaned.